This invention relates to a method and apparatus for the separation of entrained cracked products from a fluidized finely divided solid catalyst in a fluidized catalytic cracking unit (FCC). More particularly, it relates to an improved method and apparatus for separating catalyst from a catalytically cracked product in a riser reactor fluid catalytic cracking catalyst stripping zone to minimize or substantially eliminate flow of valuable cracked product to the regenerator.
The field of fluid catalytic cracking had undergone significant improvements relating both to catalyst technology and to mechanical process unit design. These advances have enabled refiners to process heavier feedstocks as well as to increase the total yields of gasoline and distillate.
By way of background, the hydrocarbon conversion catalyst usually employed in an FCC unit is preferably a high activity crystalline zeolite catalyst of a fluidizable particle size. The catalyst is transferred in suspended or dispersed phase condition generally upwardly through one or more riser conversion zones (FCC cracking zones) providing a hydrocarbon residence time in each conversion zone in the range of 0.5 to about 10 seconds, and usually less than about 8 seconds. High temperature riser hydrocarbon conversions, occurring at temperatures of at least 1000.degree. F. or higher and at 0.5 to 4 seconds hydrocarbon residence time in contact with the catalyst in the riser, are desirable for some operations before initiating separation of vapor phase hydrocarbon product materials from the catalyst. Rapid separation of catalyst from hydrocarbons discharged from a riser conversion zone is particularly desirable for restricting hydrocarbon conversion time. It is also highly desirable to strip hydrocarbon product materials from the catalyst before the catalyst enters a regeneration zone. During the hydrocarbon conversion step, carbonaceous deposits accumulate on the catalyst particles and the particles entrain hydrocarbon vapors upon removal from the hydrocarbon conversion step. The entrained hydrocarbons are removed from the catalyst in a separate catalyst stripping zone. Hydrocarbon conversion products separated from the catalyst and stripped materials are combined and passed to a product fractionation step. Stripped catalyst containing deactivating amounts of carbonaceous material, referred to as coke, is then passed to a catalyst regeneration operation.
Coke deposited on deactivated FCC catalyst together with entrained product which is carried over to the regenerator with the deactivated catalyst is referred to by those skilled in the art as "total delta carbon." For a given FCC unit design, at a fixed catalyst circulation rate, an increase in total delta carbon is accompanied by higher regenerator temperatures. Consequently, one method of limiting FCC regenerator temperature is to reduce total delta carbon by reducing carryover of cracked hydrocarbon product to the regenerator. Combustion of the valuable cracked product carried over to the regenerator not only decreases profitablility but also undesirably increases the concentration of atmospheric pollutants such as NO.sub.x in the regenerator flue gas.
Typical riser reactor fluid catalytic cracking processes mix cracking catalyst and oil feed at a ratio of about four to eight tons of catalyst per ton of oil feed. Given the substantially greater mass of the catalyst, it can well be seen that even a relatively minor improvement in catalyst stripping performance yields a valuable reduction in cracked products carryover to the regenerator.
Methods for separating catalyst particles from a gas suspension phase containing catalyst particles and hydrocarbon vapors, particularly the separation of high activity crystalline zeolite cracking catalysts, have been the subject of recent advances in the art.
Anderson et al. U.S. Pat. No. 4,043,899 discloses a method for rapid separation of a product suspension comprising fluidized catalyst particles and the vapor phase hydrocarbon product mixture, by discharging the entire suspension directly from the riser conversion zone into a cyclone separation zone. The cyclone is modified to include a separate cyclonic stripping of the catalyst separated from the hydrocarbon vapors. In the method of Anderson et al., the cyclone separator is modified to include an additional downwardly extending section comprising a lower cyclone stage. In this arrangement, catalyst separated from the gasiform material in the upper stage, slides along a downwardly sloping baffle to the lower cyclone where stripping steam is introduced to further separate entrained hydrocarbon products from the catalyst recovered from the upper cyclone. The steamed and stripped hydrocarbons are passed from the lower cyclone through a concentric pipe where they are combined with the hydrocarbon vapors separated in the upper cyclone. The separated and stripped catalyst is collected and passes from the cyclone separator by conventional means through a dipleg.
Myers et al. U.S. Pat. No. 4,070,159 provides a separation means whereby the bulk of catalyst solids is discharged directly into a settling chamber without passing through a cyclone separator. In this apparatus, the discharge end of the riser conversion zone is in open communication with the disengaging chamber such that the catalyst discharges from the riser in a vertical direction into the disengaging chamber which is otherwise essentially closed to the flow of gases. The cyclone separation system is in open communication with the riser conversion zone by means of a port located upstream from, but not near, the discharge end of the riser conversion zone. A deflector cone mounted directly above the terminus of the riser causes the catalyst to be directed in a downward path so as to prevent the catalyst from abrading the upper end of the disengaging vessel. The cyclone separator is of the usual configuration employed in a catalytic cracking unit to separate entrained catalyst particles from the cracked hydrocarbon products so that the catalyst passes through the dipleg of the cyclone to the body of the catalyst in the lower section of the disengaging vessel, and the vapor phase is directed from this vessel to a conventional fractionation unit. There is essentially no net flow of gases within the disengaging vessel beyond that resulting from a moderate amount of steam introduced to strip the catalyst residing in the bottom of the disengaging vessel. Further, the Myers et al. reference teaches a process in which hot spent catalyst is directly contacted with oxygen-containing gas withdrawn from the regenerator. The present invention, on the other hand, avoids such direct contact with its concomitant yield loss by flowing flue gas through a heat exchange conduit to indirectly heat the catalyst stripping zone.
It is also known to transfer thermal energy from the regenerator to the reactor. Gross U.S. Pat. Nos. 4,356,082 and 4,411,773 teach a fluid catalytic cracking (FCC) process and apparatus wherein the heat balance between the reactor and the regenerator of the FCC operation is partially uncoupled by transferring at least a portion of thermal energy from the reactor vessel riser to the regenerator vessel. The transfer of thermal energy results in a higher regenerating temperature. The thermal energy is recirculated to the upstream section of the reactor riser through a regenerated catalyst having higher temperature. As a result, the outlet of the reactor vessel is maintained at a substantially constant temperature (about 1000.degree. F.) and the rate of conversion of the oil feed and the octane number of gasoline produced in the process are increased.
Krug U.S. Pat. No. 4,574,044 discloses a method for increasing the overall efficiency of an FCC process by decreasing the amount of valuable product burned in the regenerator. Separation of catalyst from hydrocarbon product is enhanced by first stripping the hydrocarbon product from the catalyst and then conditioning the catalyst in the presence of steam at elevated temperatures for a period of about 1/2 to 30 minutes. The benefits of this system include a reduction in coke make.
Owen et al. U.S. Pat. No. 4,689,206 teaches an apparatus for fluid catalytic cracking (FCC) of a hydrocarbon feed in an open or closed system, which includes a multi-stage stripper system, which comprises a means for spinning a gasiform mixture of catalyst and cracked hydrocarbons exiting from a riser, a first means for stripping the spun gasiform mixture, and a means for deflecting the gasiform mixture to separate catalyst from the cracked hydrocarbons.
Commonly-assigned U.S. patent application Ser. No. 903,365 filed Sept. 3, 1986, of Herbst et al discloses a technique for improving the efficiency of a catalyst stripper section by injecting an inert gas and heating the stripper section by carrying out an exothermic reaction within the stripper.
FCC regenerators with catalyst coolers are disclosed in U.S. Pat. Nos. 2,377,935; 2,386,491; 2,662,050; 2,492,948 and 4,374,750 inter alia.